1. Field of the Invention
The present invention relates to an ink for inkjet recording comprising a non-aqueous vehicle and a coloring agent dispersed therein.
2. Description of the Related Art
Inkjet recording is a printing method in which droplets of a liquid ink high in fluidity are ejected from fine nozzles of an inkjet recording head and transferred to a recording medium such as paper. This method is advantageous in that images high in resolution and quality can be printed at high speed and with low noise using relatively inexpensive apparatuses, and so has recently become popular rapidly.
For such inkjet recording, aqueous inks are generally used, which are prepared by dissolving water-soluble dyes such as acid dyes, direct dyes, and basic dyes, or dispersing pigments in an aqueous vehicle consisting of water and glycol-based solvents or water-soluble organic solvents. However, when a recording head which has been filled with an aqueous ink is left in the atmosphere, viscosity of the ink increases due to evaporation of water from the ink, thereby causing the ejection of the ink from the nozzles to be unstable or impossible. In order to cope with this problem, a large number of methods have been proposed to stabilize the ejection of ink droplets during printing. These methods are based on so-called preliminary discharging in which inks that have increased in viscosity are previously discharged and removed to a location other than recording media. However, these methods are disadvantageous in that the ink must be consumed for the purpose other than recording.
Unlike aqueous inks, non-aqueous inks that contain no water are roughly classified into solvent-based ones and oil-based ones. The former has a vehicle consisting mainly of volatile solvents and the latter has a vehicle consisting mainly of non-volatile solvents. Especially, oil-based inks are suitable for use as inks for high-speed inkjet printers since viscosity of the inks hardly increases due to little evaporation of solvents even if the recording head is left in the atmosphere and thus frequency of cleaning or preliminary discharging can be reduced before printing.
However, conventional oil-based inks are problematic in that they cannot keep print quality constant. This is because such inks are high in permeation speed since their vehicles that are composed mainly of dispersing agents and solvents are low in surface tension and viscosity. Further, the vehicles contain non-volatile solvents as the main component thereof, and thus penetrate in the thickness direction or bleed in the horizontal direction of recording media particularly when the inks are printed on paper and other ink-permeable recording media. In addition, these vehicles tend to remain in recording media, and there often occurs seep through or strike through on paper.
In order to solve the above-mentioned problems, it might be considered to use recording media having a surface coated with pigments or the like, for example, so-called coated paper or lightly coated paper. However, such coated paper or lightly coated paper is costly but not economical, compared with non-coated paper.
In order to prevent the bleeding or strike through of vehicles, it might be considered to improve ink formulation so that vehicles do not remain in recording media. Specifically, it would be suggested to use a solvent low in boiling point. However, the solvent low in boiling point readily evaporates from the ink present in the nozzles when the recording head is left to stand in the atmosphere and then viscosity of the ink increases, thereby causing the problem of discharge failure.
Use of a solvent having a predetermined boiling point is proposed, for example, in JP-A-2002-201387, which discloses that when a solvent having a boiling point of 180 to 260 degree C. and a vapor pressure of 0.5 mmHg or less at room temperature constitutes 90% or more of the solvents, inks can be prevented from rapidly increasing in viscosity and causing nozzle clogging, thereby minimizing the probability that the solvent remains in the coated films. In addition, an inkjet ink that contains a solvent having a high boiling point of 180 degree C. or above is disclosed in JP-A-2001-164157. Furthermore, an inkjet ink that contains both a polar solvent and a non-polar solvent having a boiling point of at least 100 degree C., more preferably 200 degree C. or above is disclosed in International Publication No. WO96/12772. These proposals are effective in controlling the rapid increase in viscosity of inks, but still cause the increase of ink viscosity and the discharge failure when the recording head is left to stand for several hours or longer. Therefore, nozzle-restoring operations such as preliminary discharging and nozzle cleaning have been frequently required.